1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding a sheet (normal sheet, cut sheet, print sheet, transfer sheet, photosensitive sheet, electrostatic recording sheet, printing sheet, OHP sheet, envelope, post card, original and the like), with preventing of the skew feed of the sheet, to a sheet processing station such as a printing station, image forming station, exposure station, working station and the like in an image forming system and other various sheet using devices such as a recording system (printer), copying machine, facsimile and the like as an information output equipment such as a word processor, computer and the like.
2. Related Background Art
In the past, various device for feeding a sheet, with preventing the skew feed of the sheet, to a sheet processing station such as a printing station of a recording system have been proposed. In an exemplary sheet feeding device, the skew feed of the sheet is prevented by utilizing flexion reactive force of the sheet. That is to say, the sheet feeding means-comprises a first sheet feeding means for feeding a sheet to a sheet processing station, and a second sheet feeding means including a pair of urgingly contacted rollers disposed between the first sheet feeding means and the sheet processing station, and is so designed that a leading end of the sheet is abutted against a nip between the paired rollers of the second sheet feeding means now stopped by the normal rotation of the first sheet feeding means, and a further normal rotation of the first sheet feeding means forms a predetermined loop in the sheet between first and second sheet feeding means in opposition to the resilience of the sheet. With this arrangement, even when the sheet is skew-fed from the first sheet feeding means, the whole length of the leading end of the sheet is abutted against the nip line between the paired rollers of the second sheet feeding means, thereby registering the leading end of the sheet with the nip line. Then, when the paired rollers of the second sheet feeding means are rotated in the normal direction, the leading end of the sheet enters into the nip of the paired rollers in parallel with the nip line, with the result that the sheet is sent to the sheet processing station without the skew feed of the sheet.
On the other hand, there are conventional feeding apparatuses wherein the first and second sheet feeding means are operated as follows.
In FIG. 27, the reference numeral 303 denotes a sheet supply roller; 304 denotes a sheet stacker; 305 denotes a recording medium; 301 denotes a convey roller; and 302 denotes a driven roller. When the sheet supply roller 303 and the convey roller 301 are rotated in the normal direction, the recording medium 305 on the sheet stacker 304 is picked up. These rollers 303, 301 are rotated normally until a leading end of the recording medium 305 has passed through a nip between the convey roller 301 and the driven roller 302. Thereafter, the sheet supply roller 303 is stopped while abutting against the sheet stacker 304, and then, the convey roller 301 is rotated reversely, thereby returning the leading end of the recording medium 305 to a position upstream of the nip between the convey roller 301 and the driven roller 302 (FIGS. 28 and 29). In this condition, since a trailing end of the recording sheet 305 is pinched between the sheet supply roller 303 and the sheet stacker 304 urged against the sheet supply roller which are now stationary, the recording medium is flexed or bent between the sheet supply roller 303 and the convey roller 301 by an amount corresponding to the returning distance of the leading end of the recording medium, with the result that the leading end of the recording medium is wholly abutted against the nip line between the convey roller 301 and the driven roller 302. Thereafter, by rotating the convey roller 301 and the sheet supply roller 303 normally by a predetermined amount, the recording medium is fed to a printing position.
However, in the above-mentioned sheet feeding apparatus and its control, the leading end of the recording medium is returned toward the upstream side from the nip between the convey roller 301 and the driven roller 302, and then, is abutted against the nip, thereby preventing the skew feed of the recording medium. Thus, although the skew feed preventing ability is highly ensured in case of the cut sheet and the like a thickness of which is uniformly controlled, regarding sheets having no uniform thickness such as envelopes folded several times over and having different thickness folded portions, as shown in FIG. 30, when the leading end of the sheet is returned toward the upstream side from the nip between the convey roller 301 and the driven roller 302 and then is abutted against the nip, since positions on the leading end of the sheet are different from point to point along a line perpendicular to a plane of FIG. 30, this feeding method causes the skew feed of the sheet more noticeably than the case where the sheet is directly forwarded without returning it toward the upstream side. Further, regarding sheets having the greater thickness and high resilience, when the convey roller is rotated reversely to return the sheet toward the upstream side from the nip between the convey roller 301 and the driven roller 302, because of the high resilience of the sheet, the loop cannot be formed in the sheet between the convey roller and the sheet supply roller, but the convey roller is slipped without returning the sheet, with the result that, when the convey roller is then rotated normally by the predetermined amount to send the sheet to the print start position, the sheet will be fed excessively.
Further, in the conventional sheet feeding apparatus having the above-mentioned skew feed preventing ability, a greater space is required between the first and second sheet feeding means for permitting the formation of the predetermined loop in the sheet, because if such a space is small the sheet will be bent or folded. As a result, it was hard to make the apparatus small-sized.
Further, in a recording system of serial type wherein the main scan is effected along a direction transverse to a recording sheet feeding direction (sub scanning direction), after the recording sheet is set at a predetermined recording position, an image segment is recorded on the sheet (main scan) by a recording means (recording head) mounted on a carriage shifted along the recording sheet until the one-line recording is completed. Thereafter, the sheet is line-spaced by a predetermined amount (sub scan) and then an image segment for the next line is recorded on the recording sheet (main scan). By repeating these operations, the total image is recorded on the whole area of the recording sheet. On the other hand, in a recording system of line type wherein the recording is effected by utilizing only the sub scan for feeding a recording sheet in a sheet feeding direction, after the recording sheet is set at a predetermined recording position, an image segment for one line is recorded on the sheet en bloc. Thereafter, the sheet is advanced by a predetermined amount (pitch-feed) and then an image segment for the next line is recorded on the recording sheet en bloc. By repeating these operations, the total image is recorded on the whole area of the recording sheet.
Among these recording systems, an ink jet recording system is designed so that the recording is effected by discharging ink from a recording means (recording head) toward a recording sheet, and has advantages that the recording means can easily be made compact, an image having the high resolving power can be recorded at a high speed, the image can be recorded on a plain paper without the special treatment, the running cost is cheap, the noise can be reduced because of non-impact recording type, and a color image can easily be obtained by using plural color inks.
In particular, the ink jet recording means (recording head) for discharging the ink by utilizing thermal energy can easily be manufactured with a high dense liquid passages arrangement (discharge openings arrangement) through semi-conductor manufacturing processes such as etching, depositing, spattering and the like, thus making the recording means more compact.
The feeding mechanism (sub scanning mechanism) for the recording sheet in the above-mentioned recording systems comprises a first convey roller disposed at an upstream side of the recording head in the sheet feeding direction and a second convey roller disposed at a downstream side of the recording head in the sheet feeding direction, and is so designed that these rollers are driven synchronously with each other by a single convey motor (sub scanning motor) via a gear train. Incidentally, to establish a feeding force, each convey roller is associated with a driven roller which can be urged against the associated convey roller. Further, in order to prevent the slack of the recording sheet at the recording position, a gear ratio of the gear train is selected so that a peripheral speed of the second convey roller is greater than that of the first convey roller by a few percents or is at least equal to the peripheral speed of the first convey roller, and the feeding force obtained from the urging engagement between the second convey roller and the associated driven roller is selected to be smaller than that obtained from the urging engagement between the first convey roller and the associated driven roller. Further, in the above-mentioned gear train, the backlash is provided between gear shafts to prevent the increase in the rotational load due to the gear encroachment. Such backlash is provided between the adjacent two of all of the gear shafts.
In the feeding mechanism for the recording sheet in the above-mentioned recording systems, when a trailing end of the recording sheet is situated at an upstream side of a nip between the first convey roller and the associated driven roller in the sheet feeding direction, due to the fact that the peripheral speed of the second convey roller is greater than that of the first convey roller and the fact that the feeding force of the second convey roller is smaller than that of the first convey roller, the second convey roller is always subjected to a tension force directing toward the upstream side in the sheet feeding direction. Consequently, the driving amount of the convey motor is accurately transmitted to the second convey roller without being influenced upon the backlashes between the gear shafts, and the sheet feeding amount (sub scanning amount) itself is controlled or governed by the first convey roller, thereby performing the accurate feeding of the recording sheet (sub scan).
However, when the trailing end of the recording sheet leaves the nip between the first convey roller and the associated driven roller, the tension force acting on the second convey roller to pull the latter toward the upstream side in the sheet feeding direction is temporarily disappeared, with the result that the second convey roller is influenced upon the backlashes between the gear shafts due to the rotational inertia force of the second convey roller and is rotated by an amount greater than the normal rotation angle. Consequently, the feeding of the recording sheet (sub scan) becomes inaccurate, which results in a white blank in the recorded image, thus deteriorating the image quality. The faster the feeding speed to improve the through-put, the more this tendency is noticeable.